While it is thrilling to "read people's minds" as Gladwell puts it or to know whether someone is lying as the above websites put it less judiciously, I'm not sure that glimpsing these split-second emotions tell the whole story. Here's a personal example from my early post-high-school days:

My father made a comment to me, which aggravated me for a fraction of a second. I dismissed the emotion as quickly as it came because my father hadn't meant anything by it. So I forgot the emotion and responded in a normal, rational tone. However, my father replied in anger. I was puzzled: Why was he angry? As it so happened, the door was open at an angle that reflected my facial expression back at me. It still contained that earlier aggravation. It took me a moment to recall that flash of emotion that I had felt and dismissed. I felt no aggravation. If I had flashed aggravation and later denied it, would I be lying?

Could it be that the emotions we feel are our pre-packaged reactions, not true emotions (unless some people operate purely on pre-packaged emotions) taught to us by experience with our environment? These would help us cope with life using split-second reactions--reflexes or knee-jerk responses--much as one would remove his hand from a stove were it hot.